Catalytic partial oxidation of hydrocarbons results in a gas product containing varying proportions of hydrogen, carbon monoxide, carbon dioxide, and other components. Steam reforming of hydrocarbons also results in a gas product of similar composition. The above gas products are hereafter referred to as "synthesis gas" and the present invention is related to its production by catalytic partial oxidation of hydrocarbons. In addition, whenever the process steps of partial oxidation or steam reforming are referred to, such steps refer to the catalytic partial oxidation or steam reforming of hydrocarbons.
Commercial production of hydrogen, ammonia, and methanol depends primarily on the use of synthesis gas produced by steam reforming combined with additional downstream process steps. The steam reforming reaction is an endothermic reaction and can be represented by the reaction of methane with water, as follows: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 ( 1)
Partial oxidation, on the other hand, is an exothermic reaction, which can be represented by the reaction of methane with oxygen, as follows: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2 ( 2)
An undesirable secondary reaction may occur in catalytic partial oxidation, where oxygen may react with hydrogen to produce H.sub.2 O, and subsequently form carbon dioxide.
The formation of carbon dioxide by catalytic partial oxidation and steam reforming occurs as a secondary reaction, to those indicated as (1) or (2). That secondary reaction is the exothermic water gas shift reaction, as follows: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 3)
The selectivities of catalytic partial oxidation and steam reforming to produce the various proportions of hydrogen, carbon monoxide, carbon dioxide, and water are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed those processes generally outside the limits of economic justification. However, for the following reasons, steam reforming remains a very expensive process for production of synthesis gas as well.
To produce synthesis gas by steam reforming, high temperature heat input is primarily required at two process steps. First, sufficient steam at a high temperature and high pressure must be generated for mixing with the hydrocarbon feedstock and, secondly, the steam reforming of the steam and hydrocarbon mixture must take place at relatively high temperatures and pressures through a bed of solid catalyst. The equipment needed for these two heat transfers at high temperature and high pressure is necessarily quite expensive. The equipment for the steam reforming step is also costly because it must be adapted to permit the changing the solid catalyst when that catalyst is spent or poisoned. Heat sources appropriate for the above two process steps are typically provided by fired heaters at high, continuing utility costs, also with high fluegas NOx production consequential to the high temperatures required in the furnace firebox.
Prior art has suggested that synthesis gas production by catalytic partial oxidation could overcome some of the above disadvantages and costs of steam reforming, as follows: "Production of Methanol from Hydrocarbonaceous Feedstock", PCT Application No. PCT/US89/05369 to Korchnak et al (International Publication No. WO 90/06282)(Korchnak et al '369) and "Production of Methanol from Hydrocarbonaceous Feedstock", PCT Application No. PCT/US89/05370 to Korchnak et al (International Publication No. WO 90/06297)(Korchnak et al '370) each describe an identical process for catalytic partial oxidation. The asserted advantages of Korchnak et al '369 and '370 are relatively independent of catalyst composition, i.e. in Korchnak et al '369 the authors state that " . . . partial oxidation reactions will be mass transfer controlled. Consequently, the reaction rate is relatively independent of catalyst activity, but dependent on surface area-to-volume ratio of the catalyst." (pp. 11-12). A monolith catalyst is used with or without metal addition to the surface of the monolith at space velocities of from 20,000-500,000 hr.sup.-1. The suggested metal coatings of the monolith are selected from the exemplary list of palladium, platinum, rhodium, iridium, osmium, ruthenium, nickel, chromium, cobalt, cerium, lanthanum, and mixtures thereof in addition to metals of the groups IA, IIA, III, IV, VB, VIB, or VIIB. The catalyst surface area-to-volume ratio is in the range of 5-40 cm.sub.2 /cm.sub.3. None of the detailed embodiments indicate any preference for metal coatings, specifically or in general. The feed mixture of methane and an oxygen-containing gas to the catalyst bed must be preheated to within 200.degree. F. of the mixture's ignition temperature, but then the reaction proceeds autothermally. Steam is generally required in the feed mixture to suppress carbon formation on the catalyst. The conclusion reached by one skilled in the art is that Korchnak et al '369 and '370 attempt to solve the problem of high catalyst volumes and subsequent high costs in the use of catalytic partial oxidation by virtually eliminating the need for expensive metal coatings for the catalyst. The high catalyst volumes also require exceptional devices to attempt to evenly distribute the feed to the top of the catalyst bed, i.e., a number of tubes direct the flow of the feed gas to the top of the catalyst bed to reduce the severity of unstable operation through vapor phase combustion of the feed gas before it enters the catalyst bed.
U.S. Pat. No. 4,844,837 to Heck et al (Heck et al '837) discloses a catalytic partial oxidation method for methane using a monolith catalyst with platinum-palladium, palladium-rhodium, or platinum-rhodium coatings. There is a specific teaching "that the palladium-rhodium and platinum-rhodium combinations are rather ineffective for methane oxidation." (col. 8, 11. 28-30). The exclusion of rhodium from the monolith catalyst coating is highly preferred. The catalyst bed path required for the feed gas conversion described in this patent is calculated to be approximately one meter long.
U.S. Pat. No. 4,087,259 to Fujitani et al (Fujitani et al '259) describes a monolith catalyst with a rhodium coating to perform catalytic partial oxidation on gasoline and heavier petroleum fractions. The catalyst bed must be externally heated to maintain the reaction and the maximum space velocity is about 110,000 hr.sup.-1.